fix(clients): persist group for node-inbound clients

The client create/edit form left `group` out of the request payload, so choosing a group in the form was silently dropped (bulkAdd from the Groups page still worked because it writes the column directly). Add `group` to the payload next to `comment`.

SyncInbound also overwrote group_name unconditionally; a group set via bulkAdd is never pushed to the node, so the next node snapshot — which lacks it — wiped the column. Keep group sticky (only overwrite when the incoming value is non-empty); group is only ever set/cleared via the Groups page. Preserve comment for node clients during snapshot sync the same way. Add tests.

frontend/src/pages/clients/ClientFormModal.tsx

@@ -344,6 +344,7 @@ export default function ClientFormModal({
       reset: Number(form.reset) || 0,
       limitIp: Number(form.limitIp) || 0,
       tgId: Number(form.tgId) || 0,
+      group: form.group,
       comment: form.comment,
       enable: !!form.enable,
     };